The invention is in the field of long-life incandescent light bulbs, and three-terminal thermal switches for switching from one condition to another in response to current overload.
One type of long-life light bulb has a filament designed to operate at relatively cool temperature, and therefore has longer life than a filament designed for overall economy. However, such a light bulb has lower light output and/or consumes more power than a conventional filament designed for overall efficiency of light output, power consumption, and life. Another type of long-life light bulb has two filaments and a switch which connects one filament to operate until it burns out and then connects the other filament to operate. One example of such a light bulb is disclosed in U.S. Pat. No. 1,468,264, in which an electromagnet and spring arm connect a second filament to the power source when the first filament burns out. U.S. Pat. No. 1,838,372, discloses a switch having a heater winding around a thermostat arm for switching in a second filament when the first filament burns out. In the field of thermal switches, U.S. Pat. No. 3,629,766 discloses an overload protection switch having a fusible link which melts when there is a current overload and causes the switch to open the circuit.